C3H/HeN (responsive to Lipid-A and C3H/HeJ (not responsive to Lipid A) were primed with sheep erythrocytes so as to generate significant immunological memory. When these mice were treated subsequently with bacterial lipopolysaccharide (LPS), significant polyclonal activation of memory cells was noted only in C3H/HeN mice. This suggest that the Lipid A moiety of the LPS molecule plays an important role in the activation of memory B cells.